Resistance Characterization of Plasmopara viticola to Metalaxyl, Cymoxanil, and Cyazofamid in China

Downy mildew, caused by Plasmopara viticola, is a devastating disease that threatens global grape production, with chemical control remaining the most effective management strategy. However, the repeated application of fungicides has led to widespread resistance in P. viticola populations, while data on the resistance of P. viticola to metalaxyl (MET), cymoxanil (CYM), and cyazofamid (CYA) in China remain limited. In this study, the resistance status of P. viticola to these three fungicides was evaluated across 9 major grape-growing regions in China using leaf-disc bioassays, and potential cross- and multi-resistance patterns were assessed. The majority of isolates (127/233) exhibited either lower resistance (33.48%) or moderate resistance (21.03%) to MET based on the minimum inhibitory concentration (MIC) of 10 μg/mL and 100 μg/mL. Baseline sensitivity profiles for CYM and CYA were established as 8.69 ± 0.64 μg/mL and 0.42 ± 0.05 μg/mL, respectively, using 170 and 137 isolates. The total resistance frequency of P. viticola to CYM was 29.42% (21.18% low resistance, 8.24% moderate resistance), while that to CYA was 28.47% (18.25% low resistance, 9.49% moderate resistance, 0.73% high resistance). A weak but significant positive correlation was detected between CYM and CYA sensitivities (r = 0.193, p = 0.0196), and 13 isolates exhibited resistance to both fungicides, indicating potential multi-resistance risk. Significant regional differences in resistance profiles were observed among populations (p < 0.05), and no overall fitness penalties were detected. These findings highlight the necessity of region-specific and integrated resistance management strategies for sustainable control of grape downy mildew in China.